[analyzer] Update the checker writer manual with explanation of SVals and the link to checker callback documentation.
[analyzer] Update the checker writer manual with explanation of SVals
and the link to checker callback documentation.

SVal, SymExpr, MemRegion description is a slightly edited version of
Ted's reply to a question on cfe-dev list.

git-svn-id: ffe668792ed300d6c2daa1f6eba2e0aa28d7ec6c@146048 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/clang,apple/swift-clang,apple/swift-clang,llvm-mirror/clang,apple/swift-clang,apple/swift-clang,llvm-mirror/clang,llvm-mirror/clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,llvm-mirror/clang,apple/swift-clang,llvm-mirror/clang,apple/swift-clang,llvm-mirror/clang,llvm-mirror/clang,apple/swift-clang,llvm-mirror/clang,llvm-mirror/clang